Windows calculator, scientific mode, hex, key or click in FC66. Hit the +/- key to negate the value and note the 039A. Clear, enter 39A and hit the decimal key. See the value 922 on the display. Hit the +/- key again to reverse the sign to its original value.

Someone with a math background can probably explain it better than me, but I once noticed that if you increment 32,767 (the largest positive number that fits in a 2-byte binary field) by one you get X'8000' which is -0. Increment by one again and you get -1. Again, you get -2. In other words every increment by a +1 results in a successively smaller number. The same thing happens with 64K. Once you reach the maximum positive number, adding +1 to it results in successively smaller numbers. I guess this is because the sign of a binary number is stored in the leftmost bit. As I said, someone with a math background could I'm sure explain it better.

I don't fully understand 2s compliment but if you flip all the bits of X'FC66' which is 1111 1100 0110 0110 you get 0000 0011 1001 1001 which is X'0399'. Adding 1 you get X'039A' which is decimal 922. Reversing the sign, it becomes -922. A quicker way to get to the final result is to simply subtract 64K from the SQLCODE as shown by SQLCA. I don't know if that's more confusing or not.

One more way using the calculator is:
Repeat the procedure till 4 in the steps above...
after pasting FFFFFC66
5) press NOT button...
6) add 1....
7) I got 39A...
8) select the dec radio button now...
9) the decimal number 922...

So, the number was -922

In short: The negative of a number is stored as Two's complement which is inverting the bits from zero to 1 (1's complement and then adding 1).